I am very interested in this thread and any advice that gets put in here, I have a couple teams in the 60-71 range and based only on my limited experience, I would second Diwa's suggestion to run one team to 80, then use the gold and heirlooms from them to level the rest afterwards.

I ran instances with my 80 Pally + 4 60 Shamans and they destroyed everything, leveling very quickly to 70. At that point, the exp per level jumps way up and they are crawling now... but the upside is after 70 they can que for random dungeons with my 80 Pally as the tank.

I'm not trying to hi-jack the thread but i would like to ask for both of us, does anyone have advice and suggestions for questing 70 to 80? So far all I can find through search and reading threads here is to run all the kill/group quests and avoid the collections, but that cuts out huge parts of the quest lines in most zones... will only doing kills/group quests get my team to level 80 with only rested exp and no heirlooms?